An unspeakably ugly third-quarter performance cost the Boston Celtics Game 1 of the Eastern Conference finals. Two days later, a downright dominant performance tied their series against the Miami Heat. When both teams started to sub in their deep reserves with 6:18 left in the fourth quarter on Thursday, the Celtics led by 32 points. Against one of the NBA’s best and most physical defenses, they’d shot 38 for 70 (54.3 percent) from the field and 19 for 33 (57.6 percent) from the 3-point line, with 24 assists and only nine turnovers, scoring 143.8 points per 100 possessions on 72.6 percent true shooting.  In other words, Miami had no answers. With starters, Marcus Smart and Al Horford back after a midfoot sprain and a brief stint in the NBA’s health and safety protocols, respectively, the Celtics’ offense were otherworldly in a 127-105 Game 2 win. Smart and Horford brought what coach Ime Udoka called a “calming presence” to a team that desperately needed it. Boston’s first bucket was an open 3 from Jaylen Brown, created by Smart turning the corner, collapsing the Heat defense, and firing a perfect, crosscourt pass in mid-air, just before falling into a photographer: “They’re a heavy load team that flies out on shooters, and he did a great job driving, making the open pass, and getting guys easy shots,” Udoka said.  Smart said the goal was to “be as aggressive as you can,” but to avoid the silly stuff that doomed them in the opener: “We’re playing against a, really, really good Miami team, and when you make mistakes, they make you pay for it.” Derrick White missed the game for the birth of his child, so Smart logged 40 minutes, finishing with 24 points, 12 assists, nine rebounds, three steals, one block, and, crucially, only one turnover.   With Smart running the show again, there was less pressure on Jayson Tatum and Brown to make plays off the dribble. On one second-quarter possession, Tatum set a ball screen for Smart, then popped to the 3-point line for a catch-and-shoot 3:  That 3 was there not only because the Heat didn’t want to switch Gabe Vincent onto Tatum, but because Smart was able to keep control of the ball with P.J. Tucker swipes at it, then gets rid of it quickly. Udoka credited Smart for “dictating everything out there and getting into things.” The Celtics relentlessly attacked weak defenders, but the matchup-hunting did not turn the offense stagnant.  It took some impressive shot-making for Boston to start the game 10 for 12 from 3-point range, but it’s not as if everybody just happened to get hot. The Celtics were more purposeful in attacking Miami in transition and early-offense situations. On two plays, Horford sprinted down the court to post up a mismatch, leading to a 3 for Grant Williams and a layup for himself. On another, a well-timed screen from Williams and a pinpoint pass from Smart got Tatum a layup. The return of Horford meant the return of the lineup that started most of the second round. As soon as Grant Williams checked in for Robert Williams in the first quarter, Tatum found Grant on a back cut for a layup. A few minutes later, Brown passed out of a blitz to Grant, spotted up for a 3.  The Celtics became a championship-caliber offensive team when they started making plays like these consistently. Udoka’s message has never changed: Don’t force the issue, don’t play in a crowd, read the defense. It is much easier to do that when the offense is spaced out.  The Grant-Horford lineup “created a lot of open shots, a lot of open 3s,” Udoka said. “And [Grant] got going, but also opened up the lanes for our drivers.” Tatum, who turned the ball over six times in Game 1’s horrendous third-quarter meltdown, responded by using the Heat’s defensive aggression against them. In the third quarter he set up Smart for two catch-and-shoot 3s — first, he recognized the Heat’s early help on a drive, and then he calmly got off the ball against a blitz: Miami tried to get Boston off-balance with ball pressure and tried to junk up the game with zone defense, but the Celtics were ready for everything. Late in the third quarter, with Horford setting a high ball screen, Tatum shifted the defense and got downhill off a simple give-and-go with Smart, leading to two free throws: “Coach Udoka just kept it very simple for us [against zone],” Horford said. “He wanted us to go about it a certain way and play with pace and that’s what we did. Our guys kept making the right reads. Jayson, Jaylen, Marcus, just kind of attacking downhill and making plays for us.”  At morning shootaround on Thursday, Grant Williams said Boston knew the Heat would “try to muck up the game.” For the Celtics, he said, “it’s just about being solid and being ourselves.” They might not be immune to stretches of profoundly puzzling offense, but those are relatively rare. They believe that this mature, poised version of the team, the one that makes simple, sound decisions, is who they are. CELTICS AT HEAT PICKS AND PREDICTIONS: Heat 109, Celtics 102